Numerous applications for precision-optics components require a mirror layer which is highly reflective in the widest possible spectral range. Metals such as, for example, gold, aluminum or silver are therefore often used as a reflective coating. Of the metals, silver has the highest reflectivity from the visible to the infrared spectral range. However, silver is very susceptible to corrosion. In order to avoid the corrosion of a reflective metal layer, in particular a silver layer, a protective layer can be applied to the metal layer.
It has been found, however, that even reflective silver layers provided with a protective layer often do not have an adequate long-term stability. If, for example, silver mirrors provided with a protective layer are exposed for a relatively long time to environmental conditions with very high air humidity, or if condensation of such mirrors even occurs, liquid, in particular with dissolved corrosive substances, can pass at these locations through the protective layer to the corrosion-sensitive silver layer. A corrosion process is initiated at these locations and then propagates proceeding from this origin through the silver layer.